Zoom lens system

ABSTRACT

The present invention provides a compact zoom lens system suitable for a video camera and a digital still camera using a solid-state imaging device, having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60°, and superb optical performance. According to one aspect, in order from an object side, a first lens group has negative refractive power, a second lens group has positive refractive power, and a third lens group has positive refractive power. The front principal point of the second lens group is located to the object side of the most object side surface of the second lens group. The distance between the first and second lens group is decreased and the distance between the second and third lens group is increased upon zooming from a wide-angle end state to a telephoto end state. Predetermined conditional expressions are satisfied.

This application claims the benefit of Japanese Patent application No.2001-065242 which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system suitable for a videocamera and a digital still camera using a solid-state imaging deviceand, in particular, relates to a compact zoom lens system having a zoomratio of 3.5 or more, an angle of view in a wide-angle end state ofabout 60°, and superb optical performance.

2. Related Background Art

So far zoom lens systems disclosed in Japanese Patent ApplicationLaid-Open No. 11-52246 have been known to be those suitable for asolid-state imaging device.

However, zoom lens systems disclosed in Japanese Patent ApplicationLaid-Open No. 11-52246 have relatively small zoom ratio of about three.Therefore, it has been a problem to be insufficient zoom ratio.

SUMMARY OF THE INVENTION

The present invention is made in view of the aforementioned problem andhas an object to provide a compact zoom lens system, which is suitablefor a video camera and a digital still camera using a solid-stateimaging device, having a zoom ratio of 3.5 or more, an angle of view ina wide-angle end state of about 60°, and superb optical performance.

According to one aspect of the present invention, a zoom lens systemincludes, in order from an object side, a first lens group havingnegative refractive power, a second lens group having positiverefractive power, and a third lens group having positive refractivepower. The front principal point of the second lens group is located tothe object side of the most object side surface of the second lensgroup. The distance between the first lens group and the second lensgroup is decreased and the distance between the second lens group andthe third lens group is increased, when the state of lens grouppositions is changed from a wide-angle end state to a telephoto endstate. The following conditional expressions (1), (2), and (3) aresatisfied:

0<H 1/fw<0.6  (1)

1.0<H 2/fw<2.2  (2)

1.6<f 3/f 2<3.0  (3)

where H1 denotes the distance between the front principal point of thesecond lens group and the most object side surface of the second lensgroup, H2 denotes the distance between the rear principal point of thesecond lens group and the most image side surface of the second lensgroup, fw denotes the focal length of the zoom lens system in thewide-angle end state, f2 denotes the focal length of the second lensgroup, and f3 denotes the focal length of the third lens group.

In one preferred embodiment of the present invention, the first lensgroup and the second lens group are moved and the third lens group isfixed when the state of lens group positions is changed from thewide-angle end state to the telephoto end state. The followingconditional expression (4) is satisfied:

2.6<M/fw<3.0  (4)

where M denotes the moving distance of the second lens group along theoptical axis between the position locating the most object side and thatlocating the most image side upon zooming.

In one preferred embodiment of the present invention, the second lensgroup is composed of, in order from the object side, a first lens unithaving positive refractive power, a second lens unit having negativerefractive power, a third lens unit having negative refractive power,and a fourth lens unit having positive refractive power.

The following conditional expressions (5), (6), (7), and (8) aresatisfied:

0.6<f 21/f 2<1.0  (5)

0.7<|f 22|/f 2<2.0(f 22<0)  (6)

0.2<|f 23|/f 2<10.0(f 23<0)  (7)

1.2<f 24/f 2<3.0  (8)

where f21 denotes the focal length of the first lens unit, f22 denotesthe focal length of the second lens unit, f23 denotes the focal lengthof the third lens unit, and f24 denotes the focal length of the fourthlens unit.

In one preferred embodiment of the present invention, the first lensunit includes a cemented lens constructed by, in order from the objectside, a double convex positive lens having an aspherical surface facingto the object side cemented with a negative meniscus lens.

In one preferred embodiment of the present invention, the second lensunit includes a cemented lens constructed by, in order from the objectside, a double convex positive lens cemented with a double concavenegative lens.

In one preferred embodiment of the present invention, the third lensunit includes a negative meniscus lens having a convex surface facing tothe object side.

In one preferred embodiment of the present invention, the fourth lensunit includes a positive meniscus lens having a convex surface facing tothe object side.

In one preferred embodiment of the present invention, the first lensgroup is composed of, in order from the object side, a negative meniscuslens having a convex surface facing to the object side, a double concavenegative lens having a stronger concave surface facing to the imageside, and a positive meniscus lens having a convex surface facing to theobject side.

In one preferred embodiment of the present invention, the third lensgroup is composed of a double convex positive lens having at least oneaspherical surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a lens configuration of a zoom lens systemaccording to Example 1 of the present invention.

FIG. 2 is a drawing showing a lens configuration of a zoom lens systemaccording to Example 2 of the present invention.

FIGS. 3A-3E are graphs showing various aberrations in the wide-angle endstate according to Example 1 of the present invention.

FIGS. 4A-4E are graphs showing various aberrations in the intermediatefocal length state according to Example 1 of the present invention.

FIGS. 5A-5E are graphs showing various aberrations in the telephoto endstate according to Example 1 of the present invention.

FIGS. 6A-6E are graphs showing various aberrations in the wide-angle endstate according to Example 2 of the present invention.

FIGS. 7A-7E are graphs showing various aberrations in the intermediatefocal length state according to Example 2 of the present invention.

FIGS. 8A-8E are graphs showing various aberrations in the telephoto endstate according to Example 2 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Zoom lens systems according to Examples of the present invention aregoing to be explained in accordance with accompanying drawings. FIGS. 1and 2 are drawings showing lens configurations of zoom lens systemsaccording to Example 1 and Example 2 of the present invention,respectively.

The zoom lens system according to each Example is composed of, in orderfrom an object side, a first lens group G1 having negative refractivepower, an aperture stop AS, a second lens group G2 having positiverefractive power, and a third lens group G3 having positive refractivepower.

The first lens group G1 is composed of, in order from the object side, alens unit U11 constructed by a negative meniscus lens having a convexsurface facing to the object side, a lens unit U12 constructed by adouble concave lens having a stronger concave surface facing to an imageside than that facing to the object side, and a lens unit U13constructed by a positive meniscus lens having a convex surface facingto the object side.

The second lens group G2 is composed of, in order from the object side,a first lens unit U21 having positive refractive power, a second lensunit U22 having negative refractive power, a third lens unit U23 havingnegative refractive power, and a fourth lens unit U24 having positiverefractive power.

The first lens unit U21 is composed of a cemented lens constructed by,in order from the object side, a double convex positive lens having anaspherical surface facing to the object side cemented with a negativemeniscus lens. The second lens unit U22 is composed of a cemented lensconstructed by, in order from the object side, a double convex positivelens cemented with a double concave negative lens. The third lens unitU23 is composed of a negative meniscus lens having a convex surfacefacing to the object side. The fourth lens unit U24 is composed of apositive meniscus lens having a convex surface facing to the objectside.

The third lens group G3 is composed of a lens unit U31 constructed by adouble convex positive lens having an aspherical surface facing to theimage side.

With above described construction, when the state of lens grouppositions is changed from a wide-angle end state (a state providing theshortest focal length) to a telephoto end state (a state providing thelongest focal length), the first lens group G1 and the second lens groupG2 move while the third lens group G3 does not move. Thus, the distancebetween the first lens group G1 and the second lens group G2 isdecreased and the distance between the second lens group G2 and thethird lens group G3 is increased. The following conditional expressions(1) through (8) should be satisfied:

0<H 1/fw<0.6  (1)

1.0<H 2/fw<2.2  (2)

1.6<f 3/f 2<3.0  (3)

2.6<M/fw<3.0  (4)

0.6<f 21/f 2<1.0  (5)

0.7<|f 22|/f 2<2.0 (f 22<0)  (6)

2.0<|f 23|/f 2<10.0 (f 23<0)  (7)

1.2<f 24/f 2<3.0  (8)

where H1 denotes the distance between the front principal point of thesecond lens group and the most object side surface of the second lensgroup, H2 denotes the distance between the rear principal point of thesecond lens group and the most image side surface of the second lensgroup, fw denotes the focal length of the zoom lens system in thewide-angle end state, f2 denotes the focal length of the second lensgroup, f3 denotes the focal length of the third lens group, M denotesthe moving distance of the second lens group along the optical axisbetween the state locating the most object side and that locating themost image side upon zooming, f21 denotes the focal length of the firstlens unit, f22 denotes the focal length of the second lens unit, f23denotes the focal length of the third lens unit, f24 denotes the focallength of the fourth lens unit.

Conditional expression (1) is to realize a high zoom ratio. When theratio H1/fw falls below the lower limit of conditional expression (1),it becomes difficult to sufficiently narrow the distance between therear principal point of the first lens group and the front principalpoint of the second lens group in the telephoto end state. Accordingly,it becomes difficult to realize a high zoom ratio. On the other hand,when the ratio exceeds the upper limit of conditional expression (1),the power distribution in the second lens group becomes excessivelytelephoto state. Accordingly, it becomes difficult to realize goodcorrection of aberration.

Conditional expression (2) is to realize good correction of aberration.When the ratio H2/fw falls below the lower limit of conditionalexpression (2), the construction of the second lens group becomesrelatively simple, so that the degree of freedom to correct aberrationsbecomes small. Accordingly, in order to realize a high zoom ratio,correction of various aberrations becomes difficult. On the other hand,when the ratio exceeds the upper limit of conditional expression (2),the total lens length of the second lens group becomes large.Accordingly, the zoom lens system becomes large.

Conditional expression (3) is to realize compactness of the zoom lenssystem and to obtain an appropriate position of the exit pupil. By theway, it is desirable for a zoom lens system used for a solid-stateimaging device that the position of the exit pupil locates sufficientlyaway in the object direction from the rear principal point of the zoomlens system. When the ratio f3/f2 falls below the lower limit ofconditional expression (3), the refractive power of the second lensgroup becomes small. Accordingly, when a high zoom ratio is to berealized, the total length of the zoom lens system becomes large. On theother hand, when the ratio exceeds the upper limit of conditionalexpression (3), the position of the exit pupil tends to be close to theimage plane in the wide-angle end state, so that it is not desirable.

Conditional expression (4) is for realizing both high opticalperformance and compactness with keeping a high zoom ratio. When theratio M/fw falls below the lower limit of conditional expression (4),the production of various aberration increases. Accordingly, it becomesdifficult to realize high optical performance. On the other hand, whenthe ratio exceeds the upper limit of conditional expression (4), thezoom lens system becomes large, so that it is not desirable.

Conditional expressions (5), (6), (7), and (8) are for realizing goodcorrection of aberration of the second lens group. When these ratiosexceed or fall blow the upper limit or the lower limit of theseconditional expressions, respectively, it becomes difficult to realizegood aberration correction with keeping both high zoom ratio andcompactness of the zoom lens system.

Various values associated with Examples 1 and 2 are listed in Tables 1and 2, respectively. In each specification, f denotes the focal length,B. f. denotes the back focal length, FNO denotes the f-number, and 2Adenotes an angle of view.

In lens data, the first column is a surface number counted in order fromthe object side, the second column “r” is a radius of curvature of alens surface, the third column “d” is a distance between adjacent lenssurfaces, the fourth column “ν” is Abbe number, and the fifth column “n”is refractive index for d-line (λ=587.6 nm) where refractive index ofair (n=1.00000) is abbreviated.

In aspherical data, an aspherical surface coefficient is expressed bythe following equation:

X (y)=y²/[r·{1+(1−K·y²/r²)^(½)}]+C4·y⁴+C6·y⁶+C8·y⁸+C10·y¹⁰

where X(y) denotes the distance along the optical axis from the tangentplane on the vertex of the aspherical surface to the position of theaspherical surface at the height of y, r denotes a paraxial radius ofcurvature, K denotes the conical coefficient, and Ci denotes i-th orderaspherical surface coefficient.

In zooming data, values of the focal length and varying distances in thewide-angle end state, intermediate focal length state, and telephoto endstate are respectively shown.

In the tables for various values, “mm” is generally used for the unit oflength such as the focal length, a radius of curvature, a distancebetween the adjacent surfaces. However, since an optical systemproportionally enlarged or reduced its dimension can be obtained similaroptical performance, the unit is not necessary to be limited to “mm” andany other suitable unit can be used.

Values for conditional expressions according to each Example are listedin Table 3.

TABLE 1 [Specification] Wide-Angle Intermediate Telephoto f = 1.0001.940 3.762 B.f. = 0.350 FNO = 2.64 3.55 5.34 2A = 62.54° 32.48° 16.68°[Lens Data] r d ν n 1 3.0112 0.1578 46.58 1.80400 2 1.1780 0.4126 3−6.1499 0.1335 46.58 1.80400 4 3.1539 0.0485 5 2.0520 0.2791 23.781.84666 6 11.3500 (d 6) 7 ∞ 0.2184 Aperture Stop 8 1.3621 0.4612 55.181.66547 Aspherical Surface 9 −1.3047 0.1092 37.17 1.83400 10 −2.93250.0243 11 1.1780 0.2913 58.54 1.65160 12 −11.4357 0.1456 34.96 1.8010013 0.7642 0.0850 14 1.7353 0.2913 34.96 1.80100 15 1.1012 0.4248 161.4143 0.3034 41.49 1.57501 17 4.8747 (d17) 18 15.7767 0.2427 57.441.60602 19 −3.3516 0.0485 Aspherical Surface 20 ∞ 0.3641 64.10 1.5168021 ∞ [Aspherical Data] Surface Number: 8 K = 1.00000 C4 = −0.0479300 C6= −0.0241273 C8 = 0.0483277 C10 = −0.0661716 Surface Number: 19 K =−57.8817 C4 = −0.0860311 C6 = 0.278699 C8 = −0.251478 C10 = 0.000000[Zooming Data] Wide-Angle end Intermediate Telephoto end f 1.000001.94053 3.76213 d 6 3.07158 1.22258 0.27070 d17 0.32105 1.27394 3.11948

TABLE 2 [Specification] Wide-Angle Intermediate Telephoto f = 1.0001.942 3.762 B.f. = 0.242 FNO = 2.55 3.44 5.17 2A = 62.54° 32.36° 16.68°[Lens Data] r d ν n  1 3.1105 0.1578 46.58 1.80400  2 1.1962 0.4368  3−5.2567   0.1335 59.47 1.53996  4 3.0142 0.0412  5 1.8860 0.2762 23.781.84666  6 4.4021 (d 6)  7 ∞ 0.2184 Aperture Stop  8 1.4413 0.3922 49.321.74330 Aspherical Surface  9 −2.0224   0.1092 25.43 1.80518 10−4.5470   0.0243 11 1.4603 0.3053 60.69 1.56384 12 −4.9732   0.284934.96 1.80100 13 0.8323 0.0754 14 1.3632 0.1823 34.96 1.80100 15 1.10790.4478 16 1.6548 0.2256 47.93 1.71700 17 4.3291 (d17) 18 8.5232 0.242759.38 1.58313 19 −2.8160   0.1214 Aspherical Surface 20 ∞ 0.4000 64.141.51633 21 ∞ [Aspherical Data] Surface Number: 8 K = 1.00000 C4 =−0.0430070 C6 = −0.0114119 C8 = −0.0187353 C10 = 0.0245463 SurfaceNumber: 19 K = 1.00000 C4 = 0.120353 C6 = −0.162829 C8 = 0.458834 C10 =−0.558854 [Zooming Data] Wide-Angle end Intermediate Telephoto end f1.00000 1.94175 3.76212 d 6 3.09912 1.22825 0.26699 d17 0.35434 1.319133.18403

TABLE 3 [Values for Conditional Expressions] Example 1 Example 2 (1)H1/fw 0.405 0.445 (2) H2/fw 1.755 1.730 (3) f3/f2 2.331 1.840 (4) M/fw2.798 2.830 (5) f21/f2 0.825 0.786 (6) |f22|/f2 1.418 1.015 (7) |f23|/f22.405 5.445 (8) f24/f2 1.708 1.815

FIGS. 3A-3E through SA-SE are graphs showing various aberrations in thewide-angle end state, intermediate focal length state, and telephoto endstate according to Example 1 of the present invention, respectively.FIGS. 6A-6E through 8A-8E are graphs showing various aberrations in thewide-angle end state, intermediate focal length state, and telephoto endstate according to Example 2 of the present invention, respectively.

In graphs for various aberrations in each figure, FNO denotes thef-number, d denotes d-line (λ=587.6 nm), and g denotes g-line (λ=435.6nm). A denotes a half angle of view. In the diagrams showing sphericalaberration, astigmatism, and distortion, A denotes the maximum value ofa half angle of view. In the diagrams showing coma, A denotes a halfangle of view for each image. In the diagrams showing astigmatism, asolid line indicates a sagittal image plane and a broken line indicatesa meridional image plane.

As is apparent from the respective graphs, the zoom lens systemaccording to each Example shows superb optical performance as a resultof good corrections to various aberrations.

As described above, the present invention makes it possible to realize acompact zoom lens system, which is suitable for a video camera and adigital still camera using a solid-state imaging device, having a zoomratio of 3.5 or more, an angle of view in a wide-angle end state ofabout 60°, and superb optical performance.

Additional advantages and modification will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A zoom lens system comprising, in order from anobject side: a first lens group having negative refractive power; asecond lens group having positive refractive power; and a third lensgroup having positive refractive power; the front principal point of thesecond lens group being located to the object side of the most objectside surface of the second lens group; the distance between the firstlens group and the second lens group being decreased and the distancebetween the second lens group and the third lens group being increasedwhen the state of lens group positions is changed from a wide-angle endstate to a telephoto end state; and the following conditionalexpressions being satisfied: 0<H 1/fw<0.6 1.0<H 2/fw<2.2 1.6<f 3/f 2<3.0where H1 denotes the distance between the front principal point of thesecond lens group and the most object side surface of the second lensgroup; H2 denotes the distance between the rear principal point of thesecond lens group and the most image side surface of the second lensgroup; fw denotes the focal length of the zoom lens system in thewide-angle end state; f2 denotes the focal length of the second lensgroup; and f3 denotes the focal length of the third lens group.
 2. Thezoom lens system according to claim 1; wherein the first lens group andthe second lens group are moved and the third lens group is fixed whenthe state of lens group positions is changed from the wide-angle endstate to the telephoto end state, and the following conditionalexpression is satisfied: 2.6<M/fw<3.0 where M denotes the movingdistance of the second lens group along the optical axis between theposition locating the most object side and that locating the most imageside upon zooming.
 3. The zoom lens system according to claim 2, whereinthe second lens group is composed of, in order from the object side; afirst lens unit having positive refractive power; a second lens unithaving negative refractive power; a third lens unit having negativerefractive power; and a fourth lens unit having positive refractivepower; wherein the following conditional expressions are satisfied;0.6<f 21/f 2<1.0 0.7<|f 22|/f 2<2.0(f 22<0) 2.0<|f 23|/f 2<10.0(f 23<0)1.2<f 24/f 2<3.0 where f21 denotes the focal length of the first lensunit; f22 denotes the focal length of the second lens unit; f23 denotesthe focal length of the third lens unit; f24 denotes the focal length ofthe fourth lens unit.
 4. The zoom lens system according to claim 3,wherein the first lens unit includes a cemented lens constructed by, inorder from the object side, a double convex positive lens having anaspherical surface facing to the object side cemented with a negativemeniscus lens.
 5. The zoom lens system according to claim 3, wherein thesecond lens unit includes a cemented lens constructed by, in order fromthe object side, a double convex positive lens cemented with a doubleconcave negative lens.
 6. The zoom lens system according to claim 3,wherein the third lens unit includes a negative meniscus lens having aconvex surface facing to the object side.
 7. The zoom lens systemaccording to claim 3, wherein the fourth lens unit includes a positivemeniscus lens having a convex surface facing to the object side.
 8. Thezoom lens system according to claim 3, wherein the first lens group iscomposed of, in order from the object side; a negative meniscus lenshaving a convex surface facing to the object side; a double concavenegative lens having a stronger concave surface facing to the imageside; and a positive meniscus lens having a convex surface facing to theobject side.
 9. The zoom lens system according to claim 3, wherein thethird lens group is composed of a double convex positive lens having atleast one aspherical surface.